Method for eliminating carbon contamination of platinum-containing components for a glass making apparatus

ABSTRACT

In the formation of sheet material from molten glass, molten glass is formed in a melting furnace and transported through a precious metal delivery system to the forming apparatus. Disclosed herein is a method to mitigate carbon contamination of individual components of the precious metal delivery system prior to and/or during their use. The method involves positioning an oxygen generating material within portions of a precious metal component, and may comprise one or more heat treating steps of the component in an oxygen-containing atmosphere.

This is a continuation-in-part of U.S. patent application Ser. No.12/871,222 filed on Aug. 30, 2010, the content of which is relied uponand incorporated herein by reference in its entirety, and the benefit ofpriority under 35 U.S.C. §120 is hereby claimed.

FIELD

This invention relates to a method for ameliorating the effects ofcarbon contamination of platinum-containing components used in a glassmaking apparatus.

BACKGROUND

A glass making apparatus for delivering high quality glass in themanufacture of precision glass articles requires careful attention tothe delivery systems. Such precision products can include optical lensesand glass panels for the manufacture of display devices such astelevisions, computers, cell phones, etc.

Molten glass delivery systems for high precision products may typicallybe formed from precious metals, and usually platinum or platinum alloyssuch as a platinum rhodium alloy. Such precious metals, usually selectedfrom the platinum group of metals, have high melting temperatures, andare less likely to contribute contaminants to the molten glass (melt)flowing through these “platinum” delivery systems. In many instances,individual components of a particular platinum delivery system, a finerfor example, or a stirring vessel, are produced by joining multiplesubcomponents. For example, a cylindrical tube might be formed byrolling several flat platinum plates into semicircular segments, thenwelding the segments to form the tube. In another example, stirrers forstirring the molten glass may be formed by welding individual stirringblades to a shaft. Even the shaft may be formed from multiplecomponents.

In spite of the relatively benign behavior of platinum (or platinumalloy) when submerged within the corrosive molten glass, it has beenfound that some of these platinum components may be contributing toinadvertent contamination of the molten glass with gaseous inclusions,or blisters.

Blisters believed to originate from precious metal components, such asan apparatus for stirring molten glass, have been identified as asignificant loss issue in the manufacture of glass sheet for LCD displaysubstrates. The problem is especially prevalent during startup of amelting furnace, but has also been observed mid-campaign. Because thedefects constitute greater than about 90% CO₂, the underlying problem isbelieved to be carbon contamination of the components. The carboncontamination may be present in the components as-received from thecomponent manufacturer, or it might be introduced into the componentduring operation.

The following disclosure addresses treating individual components and/orsub-components prior to and/or during use to mitigate the formation ofthese gaseous inclusions.

SUMMARY

Disclosed herein are embodiments of methods to produceplatinum-containing articles for use in a glass making system thatproduce little or no carbon diffusion out of the article during use ofthe article (e.g. during contact of the article with molten glass).Carbon content of the article as low as several ppm may result in theformation of CO₂ gas at the interface between the platinum and moltenglass that produces bubbles in the molten material that persist,undesirably, into the final glass article. One aspect of internal carboncontamination is the introduction of carbon into sealed cavitiesproduced when an article, such as a stirring rod (stirrer), isfabricated. A number of opportunities exist for this to occur: somestirrer shafts are double walled with the space between the walls sealedby welds, sleeves are used to attach the blade assembly to the shaft andif these are also welded, a continuous cavity can be created where thestirrer blade is attached to the outer wall by the welding. The carboncan come from a variety of sources, but most commonly occurs when carboncontaining lubricants are used in the manufacture of platinum-containingsub-assemblies and assemblies. Inadequate cleaning techniques may leavea carbonaceous residue on the surface of the stirrer components beforeassembly.

Platinum-containing components are often used in delivery systems fortransporting the molten material from one location to another, or forprocessing the molten mass, such as homogenizing the material due to thehigh temperature resistant capabilities of the metal. Such articles maybe formed from platinum, or a platinum alloy, such as, but not limitedto platinum-rhodium alloys and platinum-iridium alloys. Conventionalcleaning methods, such as washing with detergents, may not remove carbonthat diffuses into the body of the platinum article. Therefore, othermethods may be employed to eliminate the carbon.

The molten glass may also be referred to as the glass melt or simply“melt”. It should be understood that glass as commonly understoodcomprises a material in an elastic state, and that although the moltenmaterial produced by the melter is not at that point truly a glass, itis capable of forming a glass upon cooling, and those skilled in the artof glass making will understand the use of the term. Thus, as usedherein, the term “molten glass” will refer to a molten fluid materialcomprised from inorganic oxide materials, which, when cooled, is capableof forming a glass.

In accordance with one embodiment, a method of making aplatinum-containing component used in a glass making system is disclosedcomprising providing a first platinum-containing metallic member and asecond platinum-containing metallic member, coating at least a portionof either one or both of the first platinum-containing metallic memberand the second platinum-containing metallic member with an oxygengenerating material (a material capable of releasing oxygen when heated,such as a multivalent oxide material, e.g. an oxide of tin, or nitratecompounds) and joining the first platinum-containing metallic member andthe second platinum-containing metallic member to form an assembly,wherein at least a portion of the oxygen generating material is disposedin an interstitial volume between the first platinum-containing metallicmember and the second platinum-containing metallic member.

The method may further comprise heating the assembly in a heat treatingstep to a temperature of at least 1450° C. for a period of time≧12 hoursin an atmosphere comprising oxygen in an amount of at least 20% byvolume.

The joining of the first platinum-containing metallic member and thesecond platinum-containing metallic member can comprise, for example,welding the first platinum-containing metallic member to the secondplatinum-containing metallic member.

In some instances a venting passage between the interstitial volume andan atmosphere external to the interstitial volume can be provided, andsealing of the vent passage after heat treating may be conducted.

Preferably, the oxygen generating material comprises an oxide of tin,but may be another material releases a gas (e.g. oxygen) when heated.Such materials may be multivalent materials, such as oxides of tin, or,for example, nitrides, such as KNO₃.

In another embodiment, a method of removing carbon contamination from aplatinum-containing article is described comprising providing a firstplatinum-containing metallic member and a second platinum-containingmetallic member, coating at least a portion of either one or both of thefirst platinum-containing metallic member and the secondplatinum-containing metallic member with a multivalent oxide material,joining the first platinum-containing metallic member and the secondplatinum-containing metallic member to form an assembly, wherein atleast a portion of the oxygen generating material is disposed in aninterstitial volume between the first platinum-containing metallicmember and the second platinum-containing metallic member, forming avent passage between the interstitial volume and an external atmosphere,heating the assembly in a heat treating step to a temperature of atleast 1450° C. for a period of time≧12 hours in an atmosphere comprisingoxygen in an amount of at least 20% by volume and sealing the ventpassage after the heating step. The coating step may comprise depositingthe oxygen generating material as a powder.

The method may further comprise stirring a molten glass material with astirring apparatus comprising the assembly.

In still another embodiment, a method of removing carbon contaminationfrom a platinum-containing article is presented comprising providing afirst platinum-containing metallic member and a secondplatinum-containing metallic member, coating at least a portion ofeither one or both of the first platinum-containing metallic member andthe second platinum-containing metallic member with tin oxide, joiningthe first platinum-containing metallic member and the secondplatinum-containing metallic member to form an assembly, wherein atleast a portion of the tin oxide is disposed in an interstitial volumebetween the first platinum-containing metallic member and the secondplatinum-containing metallic member and forming a vent passage in theassembly that connects the interstitial volume with an externalatmosphere. The tin oxide can be deposited as a powder

The method may further comprise heating the assembly in a heat treatingstep to a temperature of at least 1450° C. for a period of time≧12 hoursin an atmosphere comprising at least 20% by volume oxygen. If needed,the vent passage can be sealed after the heating step.

The method may further comprise processing a molten glass material usingthe assembly. Processing of the molten glass material may include, forexample, stirring the molten glass or fining the molten glass.

In another embodiment, a method of removing carbon contamination from aninterior volume of a platinum-containing component is disclosedcomprising providing a platinum containing component comprising a hollowinterior portion and a passage extending between an ambient atmosphereoutside the platinum containing component and the hollow interiorportion, positioning a gas supply tube into the hollow interior portionthrough the passage, flowing an oxygen-containing gas through the gassupply tube into the interior hollow portion of the platinum-containingcomponent such that a pressure within the hollow interior portion of theplatinum-containing component is greater than a pressure of the ambientatmosphere and heating the platinum containing component to atemperature greater than about 1400° C.

In still another embodiment, a method of removing carbon contaminationfrom an interior volume of a platinum-containing component is describedcomprising providing a platinum containing component comprising a hollowinterior portion and a passage extending between an ambient atmosphereoutside the platinum containing component and the hollow interiorportion positioning an oxidizing material within the hollow interiorportion and heating the platinum containing component to a temperaturegreater than about 1400° C. such that the oxidizing material reacts withreduced carbon to produce CO₂ and a metal or reduced oxide vapor such asSnO.

In still another embodiment, an apparatus for stirring a molten glassmaterial is disclosed comprising a stirrer shaft comprising platinum ora platinum alloy, the stirrer shaft defining a vent passage andincluding a wall enclosing a hollow interior portion of the stirrershaft and a gas supply tube extending through the vent passage into thehollow interior portion for supplying a gas to the interior portion ofthe stirrer shaft.

In yet another embodiment, an apparatus for stirring a molten glassmaterial is described comprising a stirrer shaft comprising platinum ora platinum alloy, the stirrer shaft defining a vent passage andincluding a wall enclosing a hollow interior portion of the stirrershaft, a support member extending from an interior portion of thestirrer shaft wall into the hollow interior portion of the stirrershaft, the support member supporting a refractory container and athermal radiation shield positioned between the support member and thevent passage. An oxidizing material is provided in the refractorycontainer.

Additional features and advantages of the invention are set forth in thedetailed description which follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as described herein. The accompanying drawingsare included to provide a further understanding of the invention, andare incorporated in and constitute a part of this specification. It isto be understood that the various features of the invention disclosed inthis specification and in the drawings can be used in any and allcombinations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the solubility of carbon in platinum as afunction of temperature.

FIG. 2 is a graph showing the minimum concentration of carbon at thesurface of a platinum component, such as an apparatus for stirringmolten glass, needed to achieve a partial pressure of CO₂ (pCO₂) equalto 1 atmosphere as a function of the partial pressure of oxygen (pO₂) atthe platinum-molten glass interface.

FIG. 3 is an elevational view in partial cross section showing anexemplary fusion downdraw process for the manufacture of glass sheet,and showing the platinum delivery system for transporting molten glassfrom the melting furnace to the forming body;

FIG. 4 is a cross sectional view of a stirring apparatus forhomogenizing the molten glass as it flows through the platinum deliverysystem;

FIG. 5 is a close up cross sectional view of a portion of a stirrershaft, showing the joining of several constituent pieces of the stirrerthat creates an interstitial volume, and the positioning of an oxygengenerating compound in the interstitial volume.

FIG. 6 is a cross sectional view of a portion of a stirrer shaft showingthe insertion of a gas supply tube through an upper vent passage of thesitter shaft into a hollow interior portion of the shaft.

FIG. 7 is a cross sectional view of a portion of a stirrer shaft showingan oxygen-producing material positioned within a hollow interior portionof the shaft such that when the shaft is heated, such as being incontact with molten glass, the oxygen-producing material producesoxygen.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of the present invention.However, it will be apparent to one having ordinary skill in the art,having had the benefit of the present disclosure, that the presentinvention may be practiced in other embodiments that depart from thespecific details disclosed herein. Moreover, descriptions of well-knowndevices, methods and materials may be omitted so as not to obscure thedescription of the present invention. Finally, wherever applicable, likereference numerals refer to like elements.

Many modern glass making systems employ precious metal components thatare used to convey and/or process a molten glass material once thematerial has been formed by melting the batch material. Typical preciousmetals include metals selected from the platinum group of metals,including platinum, rhodium, iridium, ruthenium, palladium and osmium,and alloys thereof, although the most commonly selected are platinum,rhodium and in some cases iridium for their high melting temperature andgood to fair workability.

In spite of their suitability for molten glass process equipment, suchprecious metal components suffer from several drawbacks. For example,hydrogen permeation is one well-known phenomenon, wherein watercontained in the molten glass material can disassociate into itsconstituent hydrogen and oxygen. The hydrogen diffuses through theprecious metal to the surrounding atmosphere, leaving the oxygen in themolten glass material. The oxygen forms small gas bubbles, commonlyreferred to as “blisters”, that may continue entrained throughout theforming process and end up in the finished glass product. For precisionglass products these blisters are undesirable.

An example of a precious metal component of a glass making system is aglass stirring apparatus for homogenizing the molten glass material. Thedevelopment of hollow shaft stirrers was accompanied by the introductionof a vent from the stirrer core to the atmosphere, the rationale beingthat a sealed volume would lead to unacceptable stress when the trappedgas expanded as the stirrer was brought to operating temperature. Sincethe stirrer vent system presents a somewhat tortuous path, and thestirrer shaft is quite long and narrow, exchange between the stirrercore and the atmosphere will be slow. Therefore, any carbon-containingmaterial (located either on the radiation shield or at the bottom of theshaft core) can produce a partial pressure of reduced carbon that maylead to CO2 formation on the glass side.

Another source of blisters occurs as a result of carbon that maycontaminate isolated areas of the precious metal components. As shown inFIG. 1, carbon is quite soluble in platinum, up to about 0.1% at stirreroperating temperatures, and with no intermediate phases. In addition,the diffusivity of carbon in platinum is reasonably high at stirreroperating temperatures (approximately 10⁻⁵ cm²/s).

There is also a thermo-chemical criterion that must be met for carbondiffusion to be responsible for observed CO₂ blisters. To nucleate a CO₂bubble, the partial pressure of CO₂ (pCO₂) must be greater than about 1atmosphere. FIG. 2 shows the minimum concentration in mol fraction ofcarbon in platinum needed to achieve pCO₂=1 atm and nucleate a bubble asa function of the partial pressure of oxygen in the glass melt (pO₂) at1425° C. (illustrated by curve 6), a typical stirring apparatusoperating temperature.

Shown in FIG. 3 is a side view of an exemplary glass making apparatus 10comprising melting furnace or melter 12, finer 14, stirring apparatus16, collection vessel 18, and downcomer tube 20 for supplying moltenglass to a forming body 22 for producing a thin ribbon of glass. Finer14 is connected to melter 12 through melter to finer connecting tube 24and to stirring apparatus 16 through connecting tube 26. Collectionvessel 18 is connected upstream to stirring apparatus 16 throughconnecting tube 28. A downcomer tube 20 is connected to collectionvessel 18, and supplies molten glass to inlet 30 connected to formingbody 22. Melter 12 is typically constructed from a refractory material,such as alumina or zirconia, and is supplied with batch material that ismelted by, for example, a gas flame and/or an electric current passedbetween electrodes in the melter structure. Similarly, forming body 22is also typically formed from a refractory material. In this instance,glass making apparatus 10 comprises a fusion downdraw system, so namedbecause molten glass (glass melt) delivered to the forming bodyoverflows both sides of the forming body as separate flows, then re-joinor fuse at the bottom of the forming body as the molten glass is drawndownward by pulling rollers to produce a thin, pristine ribbon of glass31. The ribbon may be cut at the bottom of the draw area into individualglass sheets. It should be noted, however, that the forming processitself may be replaced with other forming processes, as it is thedelivery system, i.e. those precious metal components between the melterand the forming body, that are the subject of the present disclosure.These components include finer 14, stirring apparatus 16, collectionvessel 18, downcomer tube 20, inlet 30 and connecting tubes 24, 26, and28, and are collectively referred to herein as the platinum system,so-called because each of the components is formed from platinum or aplatinum alloy metal such as a platinum rhodium alloy, or coated or cladwith platinum or a platinum alloy. Moreover, while the presentdisclosure is presented in the context of the exemplary platinum systemintroduced above, the principals and teaching of the present disclosureis applicable any time platinum components are assembled for use in aglass making system. In addition, the present invention is not limitedto a fusion glass making system, but may be applied to other glassmaking processes where a glass melt is formed, such as up-draw processesor float processes.

According to the exemplary fusion glass making system described above,raw batch materials 32 are sourced to the melting furnace (as indicatedby arrow 34) where heat is applied to melt the individual constituentsof the batch and form the molten glass 36. The batch materials typicallyinclude various metal oxides and other additives as required for aspecific glass composition. The melter itself is typically formed from arefractory material, for example refractory bricks. Suitable refractorymaterials include alumina or zirconia. The melting process produces,inter alia, various gases that are entrained into the molten glass andmust be removed if a quality product is to be produced from the moltenmixture. Thus, a fining step is included. For example, the molten glasscan be flowed from melter 12 through connecting tube 24 to finer 14,where the temperature of the molten glass is raised. The increasedtemperature both decreases the viscosity of the molten glass, and causescertain fining agents (e.g. multivalent compounds such as arsenic oxide,tin oxide and/or antimony oxide) included in the batch material torelease gas, e.g. oxygen. The gas released by the fining agent entersexisting bubbles, causing them to grow and therefore rise through theglass melt faster. The increased temperature also results in a decreasein the viscosity of the molten glass that allows the bubbles to risefaster. Fining is achieved when the bubbles rise to a free surface ofthe molten glass and escape from the melt.

Once the molten glass has been fined, the molten glass is flowed throughconnecting tube 26 to stirring apparatus 16 comprising stirring vessel38, stirrer 40 rotatably disposed in the stirring vessel. Molten glassflows into the stirring vessel 38 through the stirring vessel inlet 42and is stirred by stirrer 40. Stirrer 40 typically includes a stirrershaft 44 coupled to a motor 46 through a drive mechanism (e.g., chain 48and sprockets 50) and a coupler 52. Stirrer 40 also includes stirrerblades 54 arranged on the shaft such that the blades are submerged inthe molten glass during operation of the stirrer. Stirrer 40 homogenizesthe molten glass, and removes and/or dissipates cord and other anomaliestypically resulting from refractive index differences originating fromcompositional inhomogeneities. From stirring apparatus 16 the moltenglass flows from stirring vessel outlet 56 through connecting tube 28 tocollection vessel 18, and then through downcomer tube 20 to inlet 30 offorming body 22.

Each of the components of the platinum delivery system described abovemay be formed from smaller sub-components, and assembled, such as bywelding. The following description will review assembly of stirringapparatus 16 (shown in FIG. 4), and in particular the stirrer 40, but itshould be understood that the following principals can be applied toother components of the platinum system and are not limited to thestirrer or stirring apparatus.

FIG. 5 depicts a portion of stirrer 40 where a stirrer blade 54 isattached to stirrer shaft 44. Stirrer shaft 44 may, in some embodiments,be a hollow cylinder, and can include multiple layers of aplatinum-containing metal that form the wall of the hollow cylinder. Forexample, the platinum-containing metal may be a platinum rhodium (Pt—Rh)alloy, such as 80% platinum and 20% rhodium. Stirrer 40 may be formedfor example, by a powder process and then mechanically shaped into tubesand sheet to make the shafts, blades and rims that form the stirrer. Tosimplify the illustration, the stirrer shaft depicted in FIG. 5 is of asingle-wall design. Final assembly of the stirrer is typically by inertgas welding. As shown in FIG. 5, sleeve 72 is disposed about stirrershaft 44 and welded thereto at welds 74. Stirrer blades 54 are thenwelded to sleeve 72 at welds 76. As shown, an interstitial space orvolume 78 is disposed between the sleeve 72 and the outside surface ofstirrer shaft 44. Mechanical jigs used to hold the component metalpieces in place during welding may be made of graphite or siliconcarbide, and these can be a source of carbon contamination throughrubbing or impact during assembly. Other factors such as incompletecleaning or carbon-containing tramp gases in the inert welding gas mayintroduce carbon into interstitial volume 78 as well. For example,lubricants are routinely used during extrusion, rolling or pressingoperations. Carbonaceous (carbon-containing) material comprising thelubricant can be trapped in the stirrer structure between the variouslayers of platinum-containing metal of the structure. If incompletelycleaned, such lubricants can also serve as carbon sources. Evenmilligram quantities of carbon sealed into welded cavities areundesirable because of the potential for CO₂ blistering.

A simple way to prevent blister problems resulting from carbon that mayhave been introduced during stirrer assembly is to vent the cavities andheat treat the completed stirrer in air or other oxidizing atmosphere.However, the gas path to supply O₂ and remove the generated CO₂ from aconstricted space, such as between a blade sleeve and the shaft, canrequire a long heat time to completely burn out any reduced carbon.

In accordance with embodiments disclosed herein, to ensure all carbon isremoved from assembled parts, an oxygen-supplying material 80, such as amultivalent compound (e.g. SnO₂), is included in the formed cavitybefore heat treatment. As used herein, a multivalent compound is anoxide compound comprising a constituent element capable of at least twoelectronic valence states. Such compounds, for example SnO₂, whenheated, are known to change valence state and generate oxygen. It shouldbe noted that other multivalent compounds, such as Sb₂O₅ and As₂O₅, mayalso be used. However, because arsenic and antimony compounds are toxicand pose health hazards to workers assembling the components, they aretherefore not recommended.

As shown in FIG. 5, formed cavities, such as cavities formed by weldingcomponents together, can be vented by providing a vent passage 82 toprovide a path for evolved gases to exit the cavity. In the case offabricating a stirrer shaft, the shaft can be provided with a permanentvent passage 82 to the hollow shaft interior that ultimately leads to anexternal atmosphere, such as through vent 70. However, there are weldedcavities on other component parts, such as on a rim of the stirrerblade, that cannot be vented to an atmosphere outside the stirringapparatus. The released oxygen then has the potential to distort theplatinum around the cavity because of the poor mechanical strength ofthe platinum at elevated temperatures. Thus, care must be taken so thatexcessive SnO₂ does not remain in the cavity after the high temperatureoxidation step. For these cases a vent hole can be provided, such asthrough the blade rim, and then heat treating the article. The vent holecan be closed with a small weld after the article is heat treated in anoxygen-containing atmosphere.

The amount of SnO₂ that should be added to the cavity area beforewelding can be determined from the amount of carbon contaminationexpected. Lab experiments indicate that milligram quantities of graphitein a sealed platinum (or platinum alloy) cavity will produce bubbles inthe molten glass caused by carbon permeation through the platinum metal.If a milligram of carbon is trapped under a stirrer blade sleeve, forexample, it corresponds to about 6 ppm carbon by weight, calculated onthe basis of the sleeve weight. This amount would be difficult to detectby conventional means if the sleeve is analyzed after use. The amount ofSnO₂ to include in the cavity to react with 1 mg graphite would be atminimum 12.5 mg SnO₂ for complete reaction. If the cavity will also beopen to an oxidizing atmosphere, a larger quantity is not needed unlesssevere contamination is anticipated. The main function of the SnO₂ is tooxidize carbon located far from the vent. The pressure created by theevolved CO₂ will force the gas out of the cavity. In the event there isno tramp contamination in a sealed cavity, the materials are harmless tothe precious metal component. SnO₂ will react to form O₂, Sn and SnO.The elemental Sn is soluble in platinum. However, by limiting the amountof SnO₂ powder, no liquid phases will form.

The oxygen-producing material 80 may be deposited as a powder coating,and can be applied, for example, by electrophoretic deposition, with theweld area masked to prevent weld contamination by oxides.

To ensure the removal of any residual carbonaceous material, the firstassembly (e.g. joined sleeve 72 and stirrer shaft 44) can be heattreated by heating the assembly to a temperature of at least about 1450°C. for a period of time equal to or greater than about 12 hours in anatmosphere containing equal to or greater than about 20% by volumeoxygen. The atmosphere may be air. Alternatively, the atmosphere maycontain by volume ≧30% oxygen, ≧40% oxygen, ≧50% oxygen, ≧60% oxygen,≧70% oxygen, ≧80% oxygen, ≧900% oxygen or even 100% oxygen. In someembodiments, the temperature can be as high as 1600° C. or even 1650° C.However, care should be taken not to cause oxidation damage to theassembly, so the temperature and oxygen content should be appropriatelybalanced. The time period can be extended, based, among other things, onthe thicknesses of the sleeve and or shaft, to as long as, for example,72 hours or even longer. It should be noted that the heat treating stepdescribed above is distinguished from conventional annealing steps thatmay subject the platinum-containing member to a maximum temperature ofabout 1000° C. to about 1200° C. for short periods of time, on the orderof an hour or less, as such annealing steps are not sufficient to removecarbon that has been dissolved within the platinum-containing member.Once heat treatment has been completed, the vent may be plugged if thevent would be open to molten glass during operation in the glass makingprocess, or left open if the vent leads to an exterior atmosphere and istherefore not open to the molten glass.

In an embodiment shown in FIG. 6 a positive atmosphere control system inwhich a gas 84 containing oxygen, e.g. air or O₂ gas, is directed intohollow interior portion 86 of stirrer shaft 44 through a gas supply tube88 extending through upper stirrer vent passage 90. Preferably gassupply tube 88 is coaxial with the stirrer shaft. The stirrer shaftpreferably comprises a cylindrical cross sectional shape. The gas ispressurized above ambient atmospheric pressure to ensure a positive flowinto and out of stirrer interior 86. By doing this, it is not necessaryto rely on diffusion either to supply oxygen to the stirrer interior orto remove any reaction products of carbon-containing materials andoxygen. In a preferred embodiment gas supply tube 88 is machined intothe center of the high temperature alloy that comprises stirrer coupling92, e.g. a stainless steel or Inconel alloy, and the gas supply tube ismade from the same or similar material. Although gas supply tube 88supplying the O₂-containing gas will be cooled by the incoming flow ofgas, it will be heated by the return gas from hollow stirrer interior86, thus necessitating a high temperature material for the gas supplytube. Gas flow is controlled to a low level, in a range from about 0.5to about 1.5 liter/min to minimize heating of the gas supply components,including the gas supply tube itself. This flow is sufficient to give aturnover time of several minutes, which should be adequate to oxidize(burn off) any errant carbon-containing contamination in a short time.

An alternative or augmentation to a positive oxidizing gas flow is theinsertion of an oxidizing material to the hollow stirrer interior 86. Inaccordance with FIG. 7 showing a portion of a stirrer shaft 44, aplatinum support 94 is attached, such as by welding, to the inner wallsurface of shaft 44 at a location above the molten glass level but belowthe stirrer cover (i.e. in the heated region of the molten glassstirring chamber, not shown). In one embodiment a refractory container96, such as a crucible formed from alumina, and containing oxidizingmaterial 97, is placed on support 94. Oxidizing material 97 may be ametal oxide such as, for example, SnO₂. At 1425° C. the refractory ofthe crucible and the oxidizing material 97 will not form a liquid phaseand therefore will remain substantially separate. If carbon-containingmaterial is present in hollow shaft interior 86, reduced carbon vaporswill react with the oxidizing material 97, which will vaporize. Forexample, tin oxide will vaporize to produce CO₂ (reference number 98)and Sn metal or SnO. In the case of tin oxide, the tin vapors willdeposit on the inside of the shaft wall, dissolve into the platinum orplatinum alloy of the shaft and diffuse into the glass melt where theycan react to form tin oxide dissolved in the melt. The solubility of Snin platinum at stirrer operating temperatures is several percent, so bydistributing the vapors in the shaft volume the danger of platinumfailure is minimized or eliminated. The location and temperature ofoxidizing material 97 should be selected to balance an efficientreaction with carbon with the possibility that material evaporating fromthe oxygen-supplying material will condense around the upper shaft ventpassage 90 and plug the vent passage. For example, a thermal radiationshield 100 can be positioned between support 94 (including container 96and oxidizing material 97) and vent passage 90. Thermal radiation shield100 is preferably perpendicular with the stirrer shaft wall and iscircular in shape. Thermal radiation shield 100 and the upper shaftwalls minimize blockage of the vent passage by providing an interveninglower temperature surface for condensation of the tin or other vaporizedmaterial.

It should be emphasized that the above-described embodiments of thepresent invention are merely possible examples of implementations setforth for a clear understanding of the principles of the invention. Manyvariations and modifications may be made to the above-describedembodiments of the invention without departing substantially from thespirit and principles of the invention. For example, while the abovedescription has been presented in terms of a stirrer shaft, theprincipals described can be applied to other single or multi-layerplatinum-containing components of a glass making apparatus that comeinto contact with molten glass, including but not limited to single ordouble-walled tubes or pipes used to transport the molten glass from onelocation to another location, vessels for conditioning the molten glass,and sub-assemblies of certain components, such as stirrer blades coupledor uncoupled to the stirrer shaft. All such modifications and variationsare intended to be included herein within the scope of this disclosureand the present invention and protected by the following claims.

1. A method of removing carbon contamination from an interior volume ofa platinum-containing component comprising: providing a platinumcontaining component comprising a hollow interior portion and a passageextending between an ambient atmosphere outside the platinum containingcomponent and the hollow interior portion; positioning a gas supply tubeinto the hollow interior portion through the passage; flowing anoxygen-containing gas through the gas supply tube into the interiorhollow portion of the platinum-containing component such that a pressurewithin the hollow interior portion of the platinum-containing componentis greater than a pressure of the ambient atmosphere; and heating theplatinum containing component to a temperature greater than about 1400°C.
 2. The method according to claim 1, wherein the platinum-containingcomponent is in contact with a molten glass material during the heating.3. The method according to claim 1, wherein the platinum-containingcomponent is a shaft of a glass stirring apparatus.
 4. The methodaccording to claim 1, wherein the gas supply tube does not containplatinum.
 5. The method according to claim 2, wherein at least a portionof the platinum-containing component is immersed in a molten glassmaterial.
 6. A method of removing carbon contamination from an interiorvolume of a platinum-containing component comprising: providing aplatinum containing component comprising a hollow interior portion and apassage extending between an ambient atmosphere outside the platinumcontaining component and the hollow interior portion; positioning anoxidizing material within the hollow interior portion; and heating theplatinum containing component to a temperature greater than about 1400°C. such that the oxidizing material reacts with reduced carbon toproduce CO₂ and a metal vapor.
 7. The method according to claim 6,wherein the platinum-containing component is in contact with a moltenglass material during the heating.
 8. The method according to claim 6,wherein the oxidizing material is a metal oxide.
 9. The method accordingto claim 8, wherein the metal oxide is SnO₂.
 10. The method according toclaim 6, wherein the oxidizing material is contained within a refractorycontainer.
 11. The method according to claim 10, wherein the refractorycontainer comprises alumina.
 12. The method according to claim 6,wherein a thermal radiation shield is positioned between the oxidizingmaterial and an ambient atmosphere outside the platinum-containingcomponent.
 13. An apparatus for stirring a molten glass materialcomprising: a stirrer shaft comprising platinum or a platinum alloy, thestirrer shaft defining a vent passage and including a wall enclosing ahollow interior portion of the stirrer shaft; a gas supply tubeextending through the vent passage into the hollow interior portion forsupplying a gas to the interior portion of the stirrer shaft.
 14. Anapparatus for stirring a molten glass material comprising: a stirrershaft comprising platinum or a platinum alloy, the stirrer shaftdefining a vent passage and including a wall enclosing a hollow interiorportion of the stirrer shaft; a support member extending from aninterior portion of the stirrer shaft wall into the hollow interiorportion of the stirrer shaft, the support member supporting a refractorycontainer; and a thermal radiation shield positioned between the supportmember and the vent passage.
 15. The apparatus according to claim 14,wherein the refractory container contains an oxidizing material.
 16. Theapparatus according to claim 15, wherein the oxidizing material is ametal oxide.